A conventional optical phase modulation evaluating device is known as a device for evaluating an optical phase modulation signal defined as a signal modulated in phase by a data signal at a specific symbol rate in a coherent optical communication system, and adapted to perform a phase detection of an optical phase modulation signal by using a bit delay interferometer, and to evaluate an optical phase modulation characteristic of the optical phase modulation signal (see for example a patent document 1).
FIG. 16 is a diagram showing a conventional optical phase modulation evaluating device 1 disclosed in a patent document 1. As shown in FIG. 16, the conventional optical phase modulation evaluating device 1 includes a bit delay interferometer 2, a photosensitive detector (PD) 3, and a signal processing unit 4.
An optical phase modulation signal defined as a signal modulated in phase by a data signal is firstly generated as a signal to be evaluated by the conventional optical phase modulation evaluating device 1, and inputted into a bit delay interferometer 2 constituted as an optical phase detector. The bit delay interferometer 2 is constituted by a Mach-Zehnder interferometer including optical waveguides. In a signal dividing section 2b, the optical phase modulation signal from an input port 2a is divided into lights to be passed through arms 2c and 2d (the bit delay interferometer 2 includes a bit delay device 2f). A synthesizing section 2e combines the light passed through the arm 2c and the light passed through the arm 2d to synthesize light intensity conversion signals to be outputted to the PD 3 through output ports 2g and 2h. As a result, the change in phase of the optical phase modulation signal is converted to the change in light intensity of the light intensity conversion signals. The difference in phase between the light intensity conversion signals is equal to 180 degrees (π radian). The light intensity conversion signals are outputted to the PD 3 through output ports 2g and 2h. The bit delay device 2f adds, to the arm 2d, the delay of one bit with respect to the symbol rate to ensure that the arm 2d is larger in length that the arm 2c. 
The PD 3 performs a photoelectric conversion of the light intensity conversion signal from the output terminal 2g of the bit delay interferometer 2 to an electric signal. The signal processing unit 4 demodulates the electric signal from the PD 3 into the data signal. Therefore, the conventional optical phase modulation evaluating device 1 can evaluate the optical phase modulation signal on the basis of error rate and waveform obtained from the demodulated signal from the signal processing unit 4.    Patent document 1: Japanese Patent Laid-Open Publication H06-21891